Numerous observation applications require high-resolution images to be obtained, in order to identify details with small dimensions in a scene which is observed. Now, the resolution of an image which is captured with an imaging system depends on the magnification of the image forming optical system which is used, and the dimension of the photosensitive elements of the image sensor.
Using an image forming optical system which has a significant magnification has the drawback, generally, of reducing the entrance field of the system. In other words, the images which are formed and captured are each limited to a restricted portion of the scene observed. Conversely, the image of each detail of the scene is very small when an optical system with a large entrance field is used. Moreover, when the image of the detail becomes smaller than a photosensitive element of the image sensor which is used, this detail is no longer visible in the image captured.
This difficulty in obtaining high-resolution images with an entrance optical field which is not too small is even more significant for an observation in the infrared range, relative to an observation in the visible range. Indeed, the individual dimension of the photosensitive elements of an infrared image sensor is currently of the order of 12 to 15 μm (micrometer), while sensors are available for the visible light range with photosensitive elements which have individual dimensions of the order of 2 to 3 μm. For example, the capture of an image with an angular aperture of the entrance optical field of 90° (degree) and an angular resolution of 200 μrad (microradian) is directly possible with visible light with a sensor of 8000×8000 photosensitive elements. But these imaging features are not directly achievable for infrared radiation in the wavelength band of 8 to 12 μm.